The present invention relates to ferromagnetic thin film memories, and more particularly, to ferromagnetic thin film memories in which states of the memory cells based on magnetization direction are determined through magnetoresistive properties of the thin film.
Digital memories are used very extensively in computers and coputer system components, in digital signal processing systems, and in other devices based on digital circuits. In those such devices and systems where (i) the memory used must permit access to any bit stored therein randomly (a random access memory or RAM), and (ii) where such access must be accomplished in times on the order of the time taken to change states in such device or system, memories based on storage in electrical circuits in monolithic integrated circuit have become dominant However, such integrated circuit or semiconductor memories still have shortcomings with respect to what is desired in such memories Primarily they are (a) such semiconductor memories lose information upon loss of electrical power, (b) they consume electrical power continually during use, and (c) they are subject to having the information content thereof scrambled in the presence of impinging radiation
Such shortcomings can be overcome by the use of memories where bit storage is based on alternative states of magnetization in magnetic materials used in each memory cell, typically thin film materials. However, such magnetic memories have shortcomings of their own. Many ferromagnetic thin film memories used inductive sensing to determine the magnetization state of the magnetic film material used in a cell for storing a bit. This sensing scheme limits the ability to reduce cell sizes sufficiently to make a dense enough memory to be cost competitive with semiconductor memory. This limit is given effect because the signal levels inductively sensed in such magnetic memories declines with reduced thicknesses and widths for the thin film portions used in a cell to store a bit due to there then being less flux linkage to be inductively sensed. The maximum packing density of thin film memory cells providing inductively sensed output signals is not at a density sufficiently high to be competitive in cost with semiconductor memories.
Further, such magnetic memories have usually been formed on a substrate not a part of an integrated circuit. This means there were large numbers of interconnections required between the decoding circuits provided in monolithic integrated circuits and the magnetic memory storage cells leading to difficult technical problems with costly solutions.
An alternative arrangement for sensing states of magnetization in thin film magnetic material portions used in memory cells for storing bits is based on choosing a thin film ferromagnetic material which also exhibits a sufficient magnetoresistance property. Because changes in electrical resistance of such a material with the application, removal or change in magnitude of a magnetic field do not depend to first order on the dimensions of the film portion, the film portion to store a bit can be made very small to thereby improve the packing density of cells in a magnetic memory. Furthermore, such an array of cells containing film portions to store bits can be provided right on a monolithic integrated circuit surface to thereby considerably ease the making of electrical interconnections between the decoding circuits and the memory cells.
However, other problems arise when such ferromagnetic thin films used for each bit are reduced to being very small and packed very closely together on such a surface so as to be very near to one another. The magnetic situation can become much more complex with fields in one film portion serving as a bit storage site affecting neighboring storage cell film portions and vice-versa. Furthermore, a resultant magnetization intended to occur along the easy access of an anisotropic ferromagnetic film can be unstable as to direction and magnitude because of substantial demagnetizing fields occurring in a memory cell thin film portion.